Semiconductor device and method of manufacturing the same

ABSTRACT

In one embodiment, a semiconductor device includes a substrate, and a plurality of insulating layers provided on the substrate. The device further includes a plurality of electrode layers provided on the substrate alternately with the plurality of insulating layers and including metal atoms and impurity atoms different from the metal atoms, lattice spacing between the metal atoms in the electrode layers being greater than lattice spacing between the metal atoms in an elemental substance of the metal atoms.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.16/294,984 filed Mar. 7, 2019, based upon and claims the benefit ofpriority from the prior Japanese Patent Application No. 2018-173608,filed on Sep. 18, 2018, the entire contents of each are incorporatedherein by reference.

FIELD

Embodiments described herein relate to a semiconductor device and amethod of manufacturing the same.

BACKGROUND

In a case where a concave portion is formed in a film on a substrate toform a metal layer in the concave portion, if a coefficient of linearexpansion of the metal layer is greater than a coefficient of linearexpansion of the film, the substrate may be warped into a downwardlyconvex shape. It is advisable to suppress such a warp of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are cross-sectional views illustrating a method ofmanufacturing a semiconductor device of a first embodiment;

FIG. 2 is a cross-sectional view illustrating a structure of thesemiconductor device of the first embodiment;

FIGS. 3A to 4C are cross-sectional views illustrating the method ofmanufacturing the semiconductor device of the first embodiment;

FIG. 5 illustrates a graph regarding a nitrogen concentration inelectrode material layers of the first embodiment;

FIG. 6 illustrates another graph regarding the nitrogen concentration inthe electrode material layers of the first embodiment;

FIG. 7 illustrates a graph regarding an X-ray diffraction measurement ofthe electrode material layers of the first embodiment;

FIG. 8 illustrates a graph regarding annealing according to theelectrode material layers of the first embodiment;

FIGS. 9A to 9C are cross-sectional views illustrating a method ofmanufacturing a semiconductor device of a second embodiment; and

FIGS. 10A to 10C are cross-sectional views illustrating a method ofmanufacturing a semiconductor device of a modification of the secondembodiment.

DETAILED DESCRIPTION

In one embodiment, a semiconductor device includes a substrate, and aplurality of insulating layers provided on the substrate. The devicefurther includes a plurality of electrode layers provided on thesubstrate alternately with the plurality of insulating layers andincluding metal atoms and impurity atoms different from the metal atoms,lattice spacing between the metal atoms in the electrode layers beinggreater than lattice spacing between the metal atoms in an elementalsubstance of the metal atoms.

Embodiments will now be explained with reference to the accompanyingdrawings. The same reference numerals are used to designate the same orsimilar components throughout FIGS. 1A to 10C, and redundantdescriptions thereof are omitted.

First Embodiment

FIGS. 1A to 1C are cross-sectional views illustrating a method ofmanufacturing a semiconductor device of a first embodiment. An exampleof the semiconductor device of this embodiment is a three-dimensionalmemory.

First, a lower layer 2 is formed on a substrate 1, and a film stack thatalternately includes a plurality of sacrificial layers 3 and a pluralityof insulating layers 4 is formed on the lower layer 2 (FIG. 1A). Next,an upper layer 5 is formed on this film stack, and a mask layer 6 isformed on the upper layer 5 (FIG. 1A).

Examples of the substrate 1 include a semiconductor substrate, such as asilicon (Si) substrate. FIG. 1A illustrates an X direction and a Ydirection that are parallel to a surface of the substrate 1 and areperpendicular to each other, and a Z direction that is perpendicular tothe surface of the substrate 1. In the present disclosure, the +Zdirection is set as an upward direction, and the −Z direction is set asa downward direction. The −Z direction may or may not coincide with thedirection of gravity.

Examples of the lower layer 2 include insulators such as a silicon oxidefilm (SiO₂) and a silicon nitride film (SiN), and conductive layersformed between insulators. The sacrificial layers 3 are silicon nitridefilms, and the insulating layers 4 are silicon oxide films, for example.Examples of the upper layer 5 include insulators such as silicon oxidefilms and silicon nitride films and conductive layers formed betweeninsulators. Examples of the mask layer 6 include an organic hard masklayer.

Next, an opening pattern for forming a memory hole M is formed in themask layer 6 through lithography and dry etching (FIG. 1B). Then, thememory hole M that penetrates the upper layer 5, the plurality ofinsulating layers 4, the plurality of sacrificial layers 3, and thelower layer 2 is formed through dry etching using the mask layer 6 (FIG.1B). After that, the mask layer 6 is removed.

Next, a block insulator 11, a charge storage layer 12, and a tunnelinsulator 13 are formed in sequence in the memory hole M (FIG. 1C). Theblock insulator 11 is an example of a first insulator, and the tunnelinsulator 13 is an example of a second insulator. Then, the blockinsulator 11, the charge storage layer 12, and the tunnel insulator 13are removed from the bottom of the memory hole M, and a channelsemiconductor layer 14 and a core insulator 15 are formed in the memoryhole M in sequence (FIG. 1C). The charge storage layer 12 is a siliconnitride film, for example. The channel semiconductor layer 14 is apolysilicon layer, for example. Examples of the block insulator 11, thetunnel insulator 13, and the core insulator 15 include silicon oxidefilms and metal insulators.

After that, the sacrificial layers 3 are removed to form a plurality ofcavities between each insulating layer 4, and a plurality of electrodelayers are formed in the cavities. This process will be described indetail later. Furthermore, various plugs, lines, inter layerdielectrics, and the like are formed on the substrate 1. In this way,the semiconductor device of this embodiment is manufactured.

FIG. 2 is a cross-sectional view illustrating the structure of thesemiconductor device of the first embodiment.

FIG. 2 illustrates an example of the semiconductor device manufacturedby the method of this embodiment. In FIG. 2, memory cell units and astepped contact portion of a three-dimensional memory are shown. In FIG.2, the lower layer 2 is formed of a first lower insulator 2 a, asource-side conductive layer (selection gate) 2 b, and a second lowerinsulator 2 c, and the upper layer 5 is formed of a cover insulator 5 a,a drain-side conductive layer (selection gate) 5 b, a first inter layerdielectric 5 c, and a second inter layer dielectric 5 d. The coverinsulator 5 a may not be provided. Each channel semiconductor layer 14is electrically connected to a diffusion layer L in the substrate 1.Each of the sacrificial layers 3 is replaced with an electrode layer 7including a tungsten (W) layer and the like.

FIG. 2 further illustrates contact plugs 16 formed in the respectivecontact holes H of the upper layer 5. Each contact plug 16 is formed insuch a manner to be electrically connected to the correspondingelectrode layer 7.

FIGS. 3A to 4C are cross-sectional views illustrating the method ofmanufacturing the semiconductor device of the first embodiment. Theprocess for replacing the sacrificial layers 3 with the respectiveelectrode layers 7 is described below.

First, in the processes of FIGS. 1A to 1C, a columnar portion 17 thatincludes the block insulator 11, the charge storage layer 12, the tunnelinsulator 13, the channel semiconductor layer 14, and the core insulator15 is formed in each memory hole M (FIG. 3A). Next, an isolation trenchT passing through the film stack including the plurality of sacrificiallayers 3 and the plurality of insulating layers 4 and extending in the Ydirection is formed (FIG. 3A).

Then, these sacrificial layers 3 are selectively removed from theisolation trench T by wet etching with hydrofluoric acid (FIG. 3B). As aConsequence, a plurality of cavities C are formed between eachinsulating layer 4, thereby exposing the upper surface and lower surfaceof each insulating layer 4 and the side surfaces of each columnarportion 17 (block insulator 11) to the cavities C.

Sequentially, a barrier metal layer 7 a forming the electrode layers 7is formed conformally on the entire surface of the substrate 1 by CVD(Chemical Vapor Deposition) (FIG. 3C). As a consequence, the barriermetal layer 7 a is formed on the surfaces of the insulating layers 4 andthe columnar portions 17 in the cavities C and the isolation trench T.Examples of the barrier metal layer 7 a include a titanium nitride film(TiN).

Next, electrode material layers 7 b forming the electrode layers 7 isformed conformally on the entire surface of the substrate 1 by CVD (FIG.4A). As a consequence, the electrode material layers 7 b are embedded inthe cavities C via the barrier metal layer 7 a and are formed on thesurfaces of the insulating layers 4 in the isolation trench T. Examplesof the electrode material layers 7 b include a tungsten (W) layer.

Here, a coefficient of linear expansion of the W layer forming theelectrode material layers 7 b is smaller than a coefficient of linearexpansion of SiO₂ films forming the insulating layers 4. For thisreason, the substrate 1 may be warped into a downwardly convex shape(tensile direction) after the electrode material layers 7 b are formed.In this case, the substrate (wafer) 1 may not be able to flow. Thesubstrate 1 has a tendency to warp more greatly as the number of thelayers of the electrode material layers 7 b is increased.

In this embodiment, impurity atoms are therefore introduced into eachelectrode material layer 7 b from its surface after the electrodematerial layers 7 b are formed (FIG. 4B). Consequently, the electrodematerial layers 7 b are formed as impurity metal layers that contain Watoms as metal atoms and further contain impurity atoms different fromthe metal atoms. The average concentration of the impurity atoms in eachelectrode material layer 7 b of this embodiment is, for example,1.0×10²⁰ to 5.0×10²² atoms/cm³. Examples of the impurity atoms includeSi (silicon) atoms, B (boron) atoms, and N (nitrogen) atoms.

According to this embodiment, lattice spacing between W atoms in theelectrode material layers 7 b can be increased by introducing theimpurity atoms into the electrode material layers 7 b. This makes itpossible to return the direction of the stress applied to the substrate1 back to a direction causing the substrate 1 to warp into an upwardlyconvex shape (compressive direction), from a direction causing thesubstrate 1 to warp into a downwardly convex shape (tensile direction),so that the stress can be brought close to zero. As a result, the warpof the substrate 1 can be suppressed.

Here, a value of the lattice spacing between W atoms in the electrodematerial layers 7 b before introducing the impurity atoms is referred toas a first value, and a value of the lattice spacing between W atoms inthe electrode material layers 7 b after introducing the impurity atomsis referred to as a second value. In this embodiment, each electrodematerial layer 7 b before introducing the impurity atoms is a W layerformed of an elemental substance of W atoms (elemental W layer), and thefirst value is 0.2236 nm (note that, this value is obtained when W (110)surface of the W layer is used). On the other hand, each electrodematerial layer 7 b after introducing the impurity atoms is a W layercontaining the impurity atoms, and the second value is greater than0.2236 nm.

The metal atoms forming the electrode material layers 7 b may be atomsother than W atoms. In addition, the impurity atoms introduced into theelectrode material layers 7 b may be atoms other that the aforementionedones, as long as the lattice spacing between W atoms in the electrodematerial layers 7 b can be increased.

Then, the barrier metal layer 7 a and the electrode material layers 7 bare partially removed through etching (FIG. 4C). As a consequence, thebarrier metal layer 7 a and the electrode material layers 7 b in theisolation trench T and part of the barrier metal layer 7 a and part ofthe electrode material layers 7 b in the cavities C are removed throughetching.

After that, an insulator is embedded in the isolation trench T, andthereby the region of this insulator serves as an isolation region. Theisolation region is provided apart from the columnar portions 17 (blockinsulators 11). Further, various plugs, lines, inter layer dielectrics,and the like are formed on the substrate 1. In this way, thesemiconductor device of this embodiment is manufactured.

In FIG. 4B, a part of electrode material layer 7 b includes a firstportion A1 located close to the isolation trench T, a second portion A2located apart from the isolation trench T, and a third portion A3located further apart from the isolation trench T. The impurity atoms ofthis embodiment are introduced into each electrode material layer 7 bfrom the surface of the electrode material layer 7 b exposed into theisolation trench T, and therefore the concentration of the impurityatoms in the electrode material layer 7 b decreases as the distance fromthe isolation trench T increases. Consequently, in the semiconductordevice of this embodiment, each electrode material layer 7 b includesthe impurity atoms such that the electrode material layer 7 b has aconcentration gradient of the impurity atoms, and the concentration ofthe impurity atoms decreases as the distance from the isolation regionincreases.

Specifically, the first portion A1 has a high concentration of theimpurity atoms, the second portion A2 has a low concentration of theimpurity atoms, and the third portion A3 has an even lower concentrationof the impurity atoms. As for the local concentration of the impurityatoms in each electrode material layer 7 b, the minimum value of thelocal concentration is about 1.0×10²⁰ atoms/cm³ and the maximum value ofthe local concentration is about 5.0×10²² atoms/cm³. Accordingly, theimpurity atom concentration in the first portion A1 in this case isabout 5.0×10²² atoms/cm³.

Owing to such a concentration gradient, a region containing a highconcentration of the impurity atoms can be restricted within a vicinityof the isolation trench T. If the electrode material layers 7 b containa high concentration of the impurity atoms, an increase in resistance ofthe electrode material layers 7 b could be a problem. According to thisembodiment, such a concentration gradient narrows the region containinga high concentration of the impurity atoms, thereby achieving bothsuppression of the warp of the substrate 1 and reduction of theresistance of the electrode material layers 7 b. The experimentalresults show that the resistance of the electrode material layers 7 bafter introducing the impurity atoms in this embodiment can besuppressed to be less than or equal to 1.1 times the resistance of theelectrode material layers 7 b before introducing the impurity atoms.

FIG. 5 illustrates a graph regarding an impurity concentration inelectrode material layers 7 b of the first embodiment.

In FIG. 5, the impurity concentration in the electrode material layers 7b is plotted on the horizontal axis, and the lattice spacing between Watoms (lattice constant) in the electrode material layers 7 b is plottedon the vertical axis. The value on the vertical axis when the impurityconcentration is zero is the lattice constant of tungsten. According toFIG. 5, it is understood that the lattice spacing between W atoms in theelectrode material layers 7 b increases as the impurity concentrationincreases.

FIG. 6 illustrates another graph regarding an impurity concentration inthe electrode material layers 7 b of the first embodiment.

In FIG. 6, the impurity concentration in the electrode material layers 7b is plotted on the horizontal axis, and the stress applied to thesubstrate 1 is plotted on the vertical axis. According to FIG. 6, it isunderstood that the stress can be brought close to zero by adjusting theimpurity concentration to an appropriate value. For this reason, in theprocess shown in FIG. 4B of this embodiment, the impurity atoms areintroduced into the electrode material layers 7 b.

For example, in the process shown in FIG. 4B of this embodiment, theimpurity atoms are introduced into the electrode material layers 7 b byannealing the electrode material layers 7 b using gas containing theimpurity atoms. Examples of this gas include ammonia (NH₃) gas. In thiscase, it is considered that active N atoms originated from NH₃ moleculesenter the electrode material layers 7 b. Alternatively, when Si atomsare introduced into the electrode material layers 7 b, silane gas may beused as the annealing gas. Alternatively, when B atoms are introducedinto the electrode material layers 7 b, borane gas may be used as theannealing gas.

The annealing temperature for the electrode material layers 7 b ispreferably 300 to 900° C., for example. This is because the impurityatoms are hardly introduced into the electrode material layers 7 b at atemperature lower than 300° C. and a temperature higher than 900° C. istoo high for the annealing temperature.

FIG. 7 illustrates a graph regarding an X-ray diffraction measurement ofthe electrode material layers 7 b of the first embodiment.

In FIG. 7, the angle (20) and intensity of scattered X-rays obtained byirradiating the electrode material layers 7 b with X-rays arerespectively plotted on the horizontal axis and the vertical axis. Thatis, FIG. 7 illustrates angular dependence of the intensity (intensitydistribution) of the scattered X-rays.

A reference sign “51” indicates the intensity distribution beforeannealing the electrode material layers 7 b. A reference sign “S2”indicates the intensity distribution after annealing the electrodematerial layers 7 b for a short time. A reference sign “S3” indicatesthe intensity distribution after annealing the electrode material layers7 b for a long time. The gas used for annealing is ammonia gas. Theelectrode material layers 7 b are W layers, and specifically, W (110)surfaces of the W layers are used.

As shown with an arrow in FIG. 7, when annealing of the electrodematerial layers 7 b is continued, peak angles of the intensitydistributions decrease. This shows that the lattice spacing between Watoms in the electrode material layers 7 b increases by annealing. Thisfinding indicates that when the electrode material layers 7 b areannealed using ammonia gas, the impurity atoms that have entered theelectrode material layers 7 b by annealing act to increase the latticespacing between the W atoms in the electrode material layers 7 b.

FIG. 8 illustrates a graph regarding annealing of the electrode materiallayers 7 b of the first embodiment.

In FIG. 8, time for annealing the electrode material layers 7 b isplotted on the horizontal axis, and degree of the warp of the substrate1 is plotted on the vertical axis. The warp of the substrate 1 changesin a direction convex downward as the value on the vertical axis goesupward, and the warp of the substrate 1 changes in a direction convexupward as the value on the vertical axis goes downward.

Filled circles in FIG. 8 indicate the warp of the substrate 1 when theelectrode material layers 7 b having a shape shown in FIG. 4B isannealed. Open circles in FIG. 8 indicate the warp of the substrate 1when the electrode material layers 7 b are processed into a shape shownin FIG. 4C after annealing the electrode material layers 7 b having theshape shown in FIG. 4B for annealing time of the filled circles.According to FIG. 8, it is understood that the warp of the substrate 1changes in the direction convex upward as the annealing time increasesin both the filled circles and the open circles. Therefore, the processshown in FIG. 4C may be performed in the middle of the annealing of theelectrode material layers 7 b.

As has been described above, in this embodiment, the lattice spacingbetween the metal atoms in the electrode material layers 7 b areincreased by introducing the impurity atoms into the electrode materiallayers 7 b containing the metal atoms. Consequently, according to thisembodiment, the warp of the substrate 1 due to the electrode materiallayers 7 b can be suppressed.

Second Embodiment

FIGS. 9A to 9C are cross-sectional views illustrating a method ofmanufacturing a semiconductor device of a second embodiment. The methodis performed to form each contact plug 16 shown in FIG. 2.

First, in the processes of FIGS. 3A to 4C, a film stack that alternatelyincludes the plurality of electrode layers 7 and the plurality ofinsulating layers 4 is formed on the substrate 1. In FIG. 9A illustratesone of the electrode layers 7 in the film stack and the upper layer 5 onthe electrode layer 7. Next, the contact hole H passing through theupper layer 5 and reaching the electrode layer 7 is formed (FIG. 9A).

Then, a barrier metal layer 16 a forming the contact plug 16 is formedconformally on the entire surface of the substrate 1 by CVD (FIG. 9B).As a consequence, the barrier metal layer 16 a is formed on theelectrode layer 7 in the contact hole H and the surface of the upperlayer 5. Examples of the barrier metal layer 16 a include a TiN film.

Sequentially, a plug material layer 16 b forming the contact plug 16 isformed conformally on the entire surface of the substrate 1 by CVD (FIG.9B). As a consequence, the plug material layer 16 b is embedded in thecontact hole H via the barrier metal layer 16 a. Examples of the plugmaterial layer 16 b include a W layer.

Next, N atoms as the impurity atoms are introduced into the plugmaterial layer 16 b from the surface of the plug material layer 16 b(FIG. 9C). As a consequence, the plug material layer 16 b is formed asan impurity metal layer containing W atoms as metal atoms, and furthercontaining impurity atoms different from the metal atoms. The averageconcentration of the impurity atoms in the plug material layer 16 b ofthis embodiment is, for example, 1.0×10²⁰ to 5.0×10²² atoms/cm³.

According to this embodiment, lattice spacing between W atoms in theplug material layer 16 b can be increased by introducing the impurityatoms into the plug material layer 16 b. This makes it possible toreturn the direction of the stress applied to the substrate 1 back to adirection causing the substrate 1 to warp into an upwardly convex shape(compressive direction), from a direction causing the substrate 1 towarp into a downwardly convex shape (tensile direction), so that thestress can be brought close to zero. As a result, the warp of thesubstrate 1 can be suppressed. The stress applied to the substrate 1 mayapproach a value greater than zero (positive value), instead ofapproaching zero. The details of the metal atoms and the impurity atomsin the plug material layer 16 b of this embodiment are the same as thedetails of the metal atoms and the impurity atoms in the electrodematerial layers 7 b of the first embodiment.

After that, the surfaces of the barrier metal layer 16 a and the plugmaterial layer 16 b are polished by CMP (Chemical Mechanical Polishing),thereby removing portions of the barrier metal layer 16 a and the plugmaterial layer 16 b located outside the contact hole H. The CMP mayhowever be performed before introducing the impurity atoms into the plugmaterial layer 16 b. Furthermore, various plugs, lines, inter layerdielectrics, and the like are formed on the substrate 1. In this way,the semiconductor device illustrated in FIG. 2 is manufactured.

The impurity atoms of this embodiment are introduced into the plugmaterial layer 16 b from the upper surface of the plug material layer 16b, and therefore the concentration of the impurity atoms in the plugmaterial layer 16 b decreases as the distance from the upper surface ofthe plug material layer 16 b increases. Consequently, in thesemiconductor device illustrated in FIG. 2, the plug material layer 16 bof each contact plug 16 includes the impurity atoms such that the plugmaterial layer 16 b has a concentration gradient of the impurity atoms,and the concentration of the impurity atoms decreases as the distancefrom the upper surface of the plug material layer 16 b increases. In theplug material layer 16 b of each contact plug 16, a portion that has thehighest concentration of the impurity atoms is located at the uppersurface of the plug material layer 16 b.

As for the local concentration of the impurity atoms in each plugmaterial layer 16 b, the minimum value of the local concentration isabout 1.0×10²⁰ atoms/cm³ and the maximum value of the localconcentration is about 5.0×10²² atoms/cm³.

Owing to such a concentration gradient, a region containing a highconcentration of the impurity atoms can be restricted within a vicinityof the upper surface of the plug material layer 16 b. If the plugmaterial layer 16 b contains a high concentration of the impurity atoms,an increase in resistance of the plug material layer 16 b could be aproblem. According to this embodiment, such a concentration gradientnarrows the region containing a high concentration of the impurityatoms, thereby achieving both suppression of the warp of the substrate 1and reduction of the resistance of the plug material layer 16 b.

FIGS. 10A to 10C are cross-sectional views illustrating a method ofmanufacturing a semiconductor device of a modification of the secondembodiment.

In this modification, processes in FIGS. 9A and 9B are replaced with aprocess in FIG. 10A. Specifically, a plug material layer 16 b of thismodification is formed so as not to completely fill the contact hole H.Next, N atoms as the impurity atoms are introduced into the plugmaterial layer 16 b from the surface of the plug material layer 16 b(FIG. 10B).

Then, another plug material layer 16 c forming the contact plug 16 isformed conformally on the entire surface of the substrate 1 by CVD (FIG.10C). As a consequence, the plug material layer 16 c is embedded in thecontact hole H via the barrier metal layer 16 a and the plug materiallayer 16 b. Examples of the plug material layer 16 c include a W layer.After that, the impurity atoms are introduced into the plug materiallayer 16 c from the surface of the plug material layer 16 c. Examples ofthe impurity atoms are the same as in the first embodiment.

The size of each contact plug 16 is typically smaller than the size ofeach electrode layer 7. For this reason, only a small amount of theimpurity atoms can be introduced into each contact plug 16, which mayresult in unsatisfactory suppression of the warp of the substrate 1.Therefore, each contact plug 16 is formed of a plurality of plugmaterial layers as in this modification, and the impurity atoms areintroduced into each plug material layer, so that the impurity atomsintroduced into each contact plug can be increased. As a result, thewarp of the substrate 1 can be satisfactorily suppressed.

As has been described above, in this embodiment, the lattice spacingbetween the metal atoms in the plug material layer 16 b can be increasedby introducing the impurity atoms into the plug material layer 16 bcontaining the metal atoms. Consequently, according to this embodiment,the warp of the substrate 1 due to the plug material layer 16 b can besuppressed.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel devices and methods describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the devices andmethods described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. A semiconductor device comprising: a substrate; an insulator providedon the substrate; and a plug provided in the insulator and includingmetal atoms and impurity atoms different from the metal atoms, latticespacing between the metal atoms in the plug being greater than latticespacing between the metal atoms in an elemental substance of the metalatoms.
 2. The device of claim 1, wherein the metal atoms includetungsten atoms, and the impurity atoms include silicon atoms, boronatoms or nitrogen atoms.
 3. The device of claim 1, wherein aconcentration of the impurity atoms in the plug is 1.0×10²⁰ to 5.0×10²²atoms/cm³.
 4. The device of claim 1, wherein the plug includes theimpurity atoms such that the plug has a concentration gradient of theimpurity atoms.
 5. The device of claim 1, wherein a concentration of theimpurity atoms in the plug decreases as a distance from an upper surfaceof the plug increases.
 6. The device of claim 1, wherein a portionhaving a highest concentration of the impurity atoms in the plug islocated at an upper surface of the plug.